Macroporous Materials
A material whose molecular structure is permeable to fluid or gas flow is porous. These materials are frequently characterized by the size of their pores, which are voids in the material: very small pores having diameters less than 2 nanometers (nm) are called micropores; intermediate size pores having diameters between 2 and 50 nm are called mesopores; and very large pores having diameters greater than 50 nm are called macropores. Conventional porous materials have randomly distributed voids that exhibit neither shape nor size uniformity. In contrast, voids or particles possessing a periodic distribution or lattice are referred to as ordered and those having a narrow size and shape distribution are referred to as monodisperse.
Macroporous materials, those having macropores, have a wide range of applications in chemistry. In particular, macroporous polymers are useful as catalytic surfaces and supports, separation and adsorbent media, biomaterials, chromatographic materials, and thermal, acoustic and electrical insulators. In many of these areas, the utility of the porous system is a sensitive function of the internal pore diameters, their distribution and their morphology. Consequently, many synthetic approaches to creating these materials, both polymeric and inorganic, have focused on creating internal voids with monodisperse and controllable diameters.
Macroporous materials having not only monodisperse voids but also long-range three-dimensional ordering are referred to herein as ordered, monodisperse macroporous materials. Such materials are potentially valuable in optical applications. For example, materials having arrays of ordered, spherical pores whose diameters are about 500 nm act as diffractive optics for visible light. Furthermore, numerous optical applications have been suggested for well-ordered materials having long-range dielectric periodicity. These materials are potentially useful as substrates for planar waveguides; infrared filters; linear and nonlinear optics and chemical sensors; and for the control of spontaneous emission rates. These materials are traditionally formed using electron beam or photolithography techniques. Manufacturing limitations imposed by such processing have generally limited their utility to longer wavelength infrared and microwave applications and/or periodic structures of only two dimensions.
To be useful in optical technologies, an ordered, monodisperse macroporous material must be of high optical quality (no cracks or bubbles), controlled thickness, and uniform over a square centimeter or more. Although numerous methods have been developed for the fabrication of well-ordered macroporous materials—e.g., ion-track etching polymerization, chemically induced phase separation, block copolymer self-assembly, and copolymerization—these methods do not provide the long-range crystalline order and sample format required for optical applications. One promising technique for achieving long-range crystalline order involves the self-assembly of a sacrificial template to define the porous structure. Work in the last few years has demonstrated the utility of self-organizing systems as templates for the growth of a second material. These self-organized systems include surfactants, biological systems, liquid-droplet surfaces, and emulsions. Many of these systems are capable of producing macroporous materials of both polymers and inorganic oxides with pore sizes ≧50 nm. Unfortunately, however, the templates derived from these systems are polycrystalline, and the resulting samples lack the long range order and uniformity necessary for optical applications.
For example, U.S. Pat. No. 6,228,340 (“the '340 patent”) teaches the production of ceramics having spherical pores with a controllable pore size and a narrow pore size distribution. The '340 patent discloses the use of aqueous and nonaqueous emulsions as templates around which ceramics are deposited through a sol-gel process. Following the deposition, the emulsion template is removed by drying and heat treatment. Unfortunately, the macroporous ceramics disclosed in the '340 patent undergo significant shrinkage and although the pores show a “high degree of order,” there is no mention of the long-range periodicity necessary for optical applications.
U.S. Pat. No. 6,139,626 (“the '626 patent”) teaches a method for patterning materials according to a predetermined pattern by infiltrating nanocrystals into a template, sintering the nanocrystals into a monolithic structure, and then removing the template. According to the '626 patent, an appropriate template includes sub-micron spheres which self-assemble into colloidal crystals. The '626 patent also discloses selective chemical etching as a means for removing silica spheres. The '626 patent does not, however, disclose products having the long-range periodicity necessary for optical applications.
Simply put, the quest for ordered, monodisperse macroporous materials suitable for optical applications has proved unsuccessful.
Monodisperse Colloids
Colloids are materials resulting from the aggregation of small particles suspended in solution. Colloids comprised of monodisperse particles potentially offer the benefits of long-range order and periodicity because such colloids can adopt a long-range crystalline lattice. Colloids that have adopted a long-range crystalline form are referred to herein as ordered, monodisperse colloids. Monodisperse colloids possess uniform physical and chemical properties useful for the quantitative evaluation of the optical, magnetic, electrokinetic, or adsorptive behavior of colloidal matter. In addition, highly uniform colloids offer superior properties for commercial applications ranging from magnetic recording to optical pigments. When sedimented, colloids composed of monodisperse particles can form three-dimensional (3D) periodic ordered, monodisperse colloids. Existing strategies for preparing ordered, monodisperse colloids generally manipulate the chemistry of colloid formation. However, only silica and some polymer materials can be routinely prepared with the narrow size distributions required for forming monolithic high quality ordered, monodisperse colloids. Unfortunately, these colloids do not exhibit the optical, nonlinear optical, or electro-optical functionality of other materials. Consequently, ordered, monodisperse colloids exhibiting desirable optical, nonlinear optical, or electro-optical properties are difficult, if not impossible, to prepare under the prior art.